There are increasingly numerous applications wherein light influencing displays can be usefully employed. For example, light influencing displays are used to replace cathode ray tubes, as avionic information displays, as displays for digital watches, digital clocks, calculators, portable television receivers and a virtual plethora of other consumer related products.
Light influencing displays can be formed in many differing configurations using a number of different types of light influencing materials. As used herein, the term "light influencing material" is defined to mean any material which either emits light or which can be used to selectively vary the intensity, phase, direction, or polarization of light either being reflected from or transmitted through the material. Liquid crystal material is only one such material which exhibits the aforedescribed characteristics.
Generally, a liquid crystal display contains a plurality of pixels (picture elements), wherein each pixel includes a pair of electrodes (which are individually addressable by way of independent address means), and liquid crystal material disposed between the electrodes. When a voltage which exceeds the threshold voltage of the liquid crystal material is applied across the electrodes, the optical properties of the liquid crystal material switch. That is, the optical or physical properties change to provide either a light or dark display, the brightness of the display depending on the type of material used and the mode of operation of the display.
Liquid crystal materials can generally be triggered or switched by a relatively low threshold voltage. For this reason early non-active matrix pixel arrays suffered from "false positive" electrical charges, a condition in which static and background electrical charges exceed the threshold voltage of a given pixel, thereby causing unintended switching of the liquid crystal material. In this manner, a display signal may be generated when said pixel was not actually being addressed.
To alleviate the problems of background noise and cross-talk between adjacent pixels, various workers in the field, including the present inventors, have found it necessary to isolate pixel electrodes by interposing isolation devices between the pixel address means and at least one of the electrodes thereof. These isolation devices can take the form of photoresistors, thin film transistors, diodes, and a variety of other types of current blocking devices. The isolation devices are adapted to block the flow of electrical signals below a predetermined threshold voltage. Once the threshold voltage is exceeded, current flows through the isolation device, thereby charging the electrodes of the pixel and switching the liquid crystal material disposed therebetween.
Typically, liquid crystal displays are fabricated so as to include a large number of pixels arranged in an m.times.n matrix of rows and columns. Because it is necessary to employ a large number of pixels in the matrix in order to form a high resolution, large area display, multiplexing techniques are used to selectively address each pixel thereof. To that end, each of the pixels in a row are coupled together by a row address line. Likewise, each of the pixels in a column are coupled together by a column address line. In this manner, each pixel is electrically located at a unique intersection of two address lines (a row address line and a column address line) and is adapted to be individually addressed by applying a voltage potential across those two intersecting address lines.
A "passive" matrix is defined in the art as a matrix where the pixel electrodes are directly coupled to the address lines. In a passive matrix, the inherent voltage threshold characteristic of the display material is relied upon to selectively actuate of only those pixels which are addressed with a potential greater than the threshold voltage. However, in such a matrix system, pixels can experience an increased potential, which increase is due to the fact that they are coupled to one of the address lines to which the potential was increased; but said pixels may not be activated because the potential increase is below the threshold voltage necessary to switch the liquid crystal material of the pixel. A pixel will only be activated when the potential across the electrodes thereof is above the threshold voltage, i.e., when both address lines are energized. Further, it is well known that the number of pixels which can be employed in passive matrix liquid crystal displays have contrast and speed limitations, which are both dependent, in part, on the finite sharpness of the threshold voltage characteristics of the liquid crystal material.
In order to achieve high resolution, high contrast and high speed in liquid crystal displays having a large number of pixels, active matrix displays must be used. Active matrix displays, as discussed hereinabove, employ one or more isolation devices at each pixel so as to provide improved threshold voltage sharpness at each pixel, thereby enhancing isolation between the pixels electrically connected to common address lines. A number of different types of terminal isolation devices have been used to provide the required isolation. As used herein, the term "isolation device" will refer to any device which enhances the ability of one pixel to be addressed (switched) without switching or adversely affecting other pixels sharing a common address line. Such isolation devices include threshold devices, for example, one or more diodes arranged in various configurations, M-I-M structures, photoresistors, thin film transistors and other current control devices.
Some two terminal isolation devices, such as diodes and certain configurations of three terminal devices are single polarity (unipolar) devices. That is, single polarity devices can be turned on in only one direction or polarity. Other two terminal devices, such as diode rings, M-I-M (metal-insulator-metal) devices, n.sup.+ -i-n.sup.+ threshold isolation devices, and otherwise configured three terminal devices, such as thin film transistors, are dual polarity (bipolar) devices which can be turned on so as to conduct current in either one of two directions therethrough.
All of these isolation devices provide a more precise voltage threshold then that provided by the light influencing material itself. A precise voltage threshold results in a smaller variance in the voltage required to switch a pixel from an "off" condition to an "on" condition. Since isolation devices significantly reduce, if not totally eliminate the effects of "cross talk" and "noise", thereby providing for a greater number of pixels to be addressed by any given address line.
Liquid crystal displays which can be manufactured with high yields, utilizing diodes as the isolation devices are disclosed in commonly assigned, co-pending U.S. patent application Ser. Nos. 573,004 and 675,941 each entitled "Liquid Crystal Displays Operated By Amorphous Silicon Alloy Diodes", and filed in the names of Zvi Yaniv, Vincent D. Cannella, Gregory L. Hansell and Louis D. Swartz, on Jan. 23, 1984 and Dec. 3, 1984, respectively, which applications are incorporated herein by reference. As disclosed therein, the diodes, employed as switching devices, can be formed with reduced precision photolithography and with significantly fewer process steps then required to form other types of isolation devices, such as transistors.
The displays disclosed in the aforementioned co-pending U.S. Applications rely upon the inherent pixel capacitance to provide charge retention and maintain a pixel in a desired optical condition. As mentioned hereinabove, pixel capacitance results from the capacitance created by a pair of charged electrodes having liquid crystal material disposed therebetween. Inherent pixel capacitance, relative to the overall conductance of the light influencing material (and any other leakage paths) available to discharge the stored charge, determines the length of time a desired voltage above the minimum threshold voltage level can be held across the liquid crystal display material of a pixel (the pixel time constant). It is in this manner, that the pixel is maintained in a high voltage optical condition after the potentials are applied and during the time in which the other pixels of the display are addressed. However, it is desirable that the total capacitance of the pixel be increased so that leakage paths which can slowly discharge the pixel are insufficient to initiate readings of the presence of false optical conditions existing across at least some picture elements.
Adding additional or auxiliary capacitance to increase the total amount of charge which may be stored prior to discharge (i.e., to increase the time for pixel self discharge) is difficult. This is because the added capacitance must be applied electrically in parallel with the pixel capacitance across the electrodes, requiring an electrical connection through the liquid crystal material (which separates those electrodes). The addition of auxiliary capacitance is further structurally complicated by the fact that displays are usually electrically and structurally equipped with addressing circuitry on both electrode planes.
An improved active matrix display having all of the addressing electronic circuitry, including isolation devices on one substrate or pixel electrode plane of the display, is disclosed in U.S. patent application Ser. No. 4,589,733 entitled "Displays In Sub-Assembly Having Improved Pixel Electrodes", issued on May 20, 1986 in the names of Zvi Yaniv, Yair Baron, Vincent D. Cannella and Gregory L. Hansell, the disclosure of which is incorporated herein by reference. The displays disclosed by Yaniv, et al include a plurality of pixels, with each pixel including a first electrode having a pair of spaced apart side-by-side electrode portions on a first electrode plane and a second electrode spaced from and facing the first electrode portions on a second electrode plane. The second electrode is electrically insulated from all external circuit connections and from all other pixel electrodes. Liquid crystal material is disposed between the first electrode portions and the second electrode. Displays of this type exhibit decreased electronic complexity because all of the addressing lines are formed on the first electrode plane i.e., the electrode plane carrying first pixel electrode. In accordance with the preferred embodiment, the address lines are coupled to each portion of the first electrode by one or more devices which are adapted to provide pixel isolation.
An improved display is disclosed in commonly assigned and co-pending U.S. patent application Ser. No. 639,001, filed Aug. 8, 1984 in the name of Vincent D. Cannella for "Displays And Sub-Assemblies Having Optimized Capacitance", the disclosure of which is incorporated herein by reference. As disclosed therein, an auxiliary pixel capacitance is provided by the addition of a third auxiliary capacitance electrode. The third auxiliary capacitance electrode is spaced from and faces the first electrode portions on the side of the first electrode opposite the liquid crystal display material. The third, auxiliary capacitance electrode provides an auxiliary capacitance that is in parallel with the inherent pixel capacitance. In this manner, the capacitances, being in parallel, add and the total pixel RC time constant can be materially increased. Commonly assigned U.S. Pat. No. 4,728,802, filed Oct. 9, 1986, and issued Mar. 1, 1988 in the name of Yair Baron for "Liquid Crystal Display Having Pixels With Auxiliary Capacitance", the disclosure of which is incorporated herein by reference, provides a larger auxiliary capacitance disposed electrically in parallel with the pixel capacitance in a given display.
However, the effectiveness of providing the aforedescribed auxiliary capacitance is limited by the ability to effectively and reliably interconnect the auxiliary capacitance electrode in parallel with the inherent pixel capacitance. Heretofore, attempts to interconnect the auxiliary capacitance electrode with the pixel have been hampered by the space constraints presented by operating between the sheets of substrate. These constraints often resulted in shorts and other performance degrading defects. It is to the end of economically and reliably interconnecting auxiliary capacitance in electrical parallel with the pixel capacitance that the instant invention is directed.